This patent specification relates to the field of optical fibers. More particularly, it relates to a method of fabricating a microstructured optical fiber.
As the world""s need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers. Conventional optical fibers have a solid cross-section and are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
Conventional optical fiber fabrication techniques generally involve the formation of a cylindrical preform having the desired refractive index profile. The preform is then heated and drawn into a thin fiber. See Hecht, Understanding Fiber Optics, Prentice-Hall (1999), which is incorporated by reference herein, at pp. 108-115; see also Keiser, Optical Fiber Communication, 2nd Ed., McGraw Hill (1991), which is incorporated by reference herein, at pp. 63-68. The preform is commonly fabricated by the longitudinally concentric formation of a fluffy fused-silica soot generated by reacting SiCl4 (along with GeCl4 when used as a dopant) with oxygen to generate SiO2, i.e. silica (along with GeO2 if the silica is doped). Many approaches have been developed for accomplishing and controlling the soot formation, including inside vapor deposition, outside vapor deposition, and vapor axial deposition. The desired refractive index profile is achieved through control of the chemicals used during the soot formation process. Heating melts the soot, which then condenses into a glass. The preform, which is commonly about 10 cm in diameter, is then mounted vertically in a large drawing tower, heated at the bottom, and drawn out into a thin fiber of about 100 xcexcm-130 xcexcm in diameter. In the 100 xcexcm case, the drawing process thus reduces the diameter of the preform by a factor of 1000. Accordingly, the final optical fiber is 10002=1 million (106) times longer than the preform, with each micrometer of preform turning into one meter of optical fiber.
Special difficulties arise, however, in the fabrication of microstructured optical fibers disclosed, for example, in copending Ser. Nos. 09/591,474, 09/781,344, and 09/781,352 due to the nature of the void patterns therein. FIG. 1 illustrates an example of a microstructured optical fiber 100 disclosed in copending Ser. No. 09/781,352 in which 50.0% of the cladding cross-sectional area is occupied by voids and 50.1% of the core cross-sectional area is occupied by voids. Optical fiber 100 comprises a core region 102 made of a core material 106 and a cladding region 104 made of a cladding material 110, core material 106 having an index of refraction n1 higher than an index of refraction n2 of the cladding material 110, the effective index of refraction of the core region 102 being slightly higher than the effective index of refraction of the cladding region 104. Preferably, void sizes are substantially less than the wavelength of light being propagated, e.g. less than 10% of the wavelength. Identical first patterns of circular voids 108 and 112 are formed in the core and cladding materials, respectively, having an exemplary diameter of 100 nm and an average center to center spacing of 125 nm. A second pattern of smaller voids 109 is also formed in the core region, e.g., one small void for each larger void, each small void having a diameter of 4.5 nm. Although some preferred embodiments of the microstructured optical fiber use core and cladding materials having the same index of refraction, the optical fiber 100 in FIG. 1 represents a general case in which there is different doping in the core and cladding.
Proposals have been made for fabricating optical fibers having voids in their cross-sections, one such proposal being disclosed in U.S. Pat. No. 5,802,236 (xe2x80x9cthe ""236 patentxe2x80x9d), which is incorporated by reference herein. The method discussed in the ""236 patent generates a preform by bundling hollow silica capillary tubes around a center silica glass rod, being sure to physically arrange them in a scaled version of the ultimate desired pattern. One or more silica overcladding tubes is then placed around the entire bundle and melted around it to produce the desired preform. The preform is then drawn using conventional techniques to generate the optical fiber.
Importantly, however, the mechanical bundling process of the ""236 patent and similar methods are not readily scalable to the number and precision of voids in the index-guiding microstructured optical fiber 100 of FIG. 1. For example, it is readily calculated that, for an exemplary core diameter of 25 xcexcm and an exemplary cladding diameter of 100 xcexcm, there are about 500,000 of the larger 100 nm holes and about 31,250 of the smaller holes in the optical fiber of FIG. 1. In general, prior art processes such as those of the ""236 patent that are designed to form tens, or perhaps hundreds, of voids in the optical fiber are not well-suited for fabricating the index-guiding microstructured optical fiber 100 of FIG. 1, in which up to several hundred thousand or more voids are required.
Accordingly, it would be desirable to provide a method for fabricating a microstructured optical fiber having a large number of longitudinal voids formed therein.
It would be further desirable to provide a method for fabricating a microstructured optical fiber in which the voids can be formed in any of a variety of shapes and patterns.
It would be still further desirable to provide a method for fabricating a microstructured optical fiber in which the voids can be designed in complex patterns, including even arbitrarily complex patterns, yet fabricated to a high degree of precision.
In accordance with a preferred embodiment, a method of fabricating a microstructured optical fiber is provided in which a plurality of solid wafers are generated corresponding to longitudinally consecutive portions of the optical fiber, separately etched with void patterns in a lithographic process, and bonded together into a preform. The preform is then drawn to form the optical fiber. The lithographic process used to form the void patterns in the wafers may be any of several processes currently or prospectively used in very large scale integrated circuit (VLSI) fabrication. Such lithographic process may be used because the wafers comprise silica glass or other material common in VLSI devices and, in accordance with a preferred embodiment, are generated with nominal thicknesses highly amenable to such fabrication methods.
Any of several methods may be used to initially generate the solid wafers in accordance with a preferred embodiment. In one preferred embodiment, a preliminary preform having the desired material refractive index profile is generated using conventional preform fabrication methods. The wafers are then mechanically sliced from the preliminary preform.
In another preferred embodiment, the wafers are formed by a flame hydrolysis process similar to processes used in planar waveguide technology, which may be controlled to yield the desired refractive index profile for each wafer. In another preferred embodiment, the wafers are grown by forming a layer of SiO2 using a chemical vapor deposition process or similar semiconductor fabrication process known to grow SiO2. In still another preferred embodiment, the wafers may be formed by oxidizing silicon (Si) to form a layer of SiO2 thereon, in a process similar to a known semiconductor fabrication processes. The refractive index profile of each wafer may be initially achieved and/or modified by a hybrid chemical/lithographic process.
The wafer bonding process is similar to SiO2xe2x80x94SiO2 bonding processes known in the semiconductor fabrication field. Depending on which of the above methods is used to generate the wafers, each wafer lies on its own silicon substrate, or if no such substrate is present, one is attached. A first and second wafer are bonded together, and the silicon substrate remaining on top of the two-element stack is removed. A third wafer is bonded to the two-element stack, and the silicon substrate remaining on top of the three-element stack is removed, and so on. After all wafers have been added, the bonded stack constitutes the desired preform.
Advantageously, highly sophisticated lithographic techniques for forming complex patterns in silica (SiO2) are known in the field of VLSI technology. Because such lithographic techniques can used to form the void patterns and/or spatially dope the wafers, the resulting optical fiber can have a cross-section of highly complex patterns and yet be fabricated to a high degree of precision.